1. Technical Field of the Invention
This invention pertains generally to methods and systems for treating wastewater, and more specifically, methods and systems for completely recycling gas well hydraulic fracture flow-back wastewaters.
2. Background of the Invention
Water is essential to the oil and gas industry. Not only is it used in large volumes during the initial drilling process to carry dirt and rock to the surface, and to cool and lubricate the drilling equipment, water is essential in hydraulic fracture. Hydraulic fracture, or fracking, is a commonly used process to maximize the extraction of underground resources such as oil and natural gas. Fluid is pumped at a high pressure into a geological formation to crack or fracture the rock structures possessing hydrocarbons. The fracture width is maintained using a proppant such as sand, ceramic, or other particulates, thus allowing hydrocarbons to flow to the surface of the well. In addition to fracturing the rock, water also serves as a transport medium for the proppant. Accordingly, the hydraulic fracture process requires millions of gallons of water per well. A single gas well in the Marcellus Shale geographic area, for example, uses an average of 100,000 gallons of water to drill the well and 5.5 million gallons of water for hydraulic fracture. If the water is transported by tanker truck with a standard carrying capacity of 5,000 gallons, it would take 1,120 one-way trips (2,240 round trips), with each tanker carrying a load of 41,700 pounds, to transport water to the drilling site. A recent count by the Carnegie Museum of Natural History (May, 2012) found that 9,848 gas wells have been drilled in the Marcellus Shale geographic area. At an average usage rate of 5.6 million gallons of water per well, that equates to 55 billion gallons of water.
After hydraulic fracture is complete, the internal pressure of the geologic formation causes the injected water to rise to the surface where it may be recovered and stored in tanks or pits prior to disposal or recycling. Recovered water, commonly referred to as produced water or flow-back water, carries with it numerous chemicals added during the hydraulic fracturing process in addition to salts (chlorides, bromides, and sulfides of calcium, magnesium, and sodium), metals (barium, manganese, iron, and strontium, among others), bacteria and hydrocarbons leached from the geologic formation. On average, seven to nine barrels of contaminated flow-back water may be generated during the production of a single barrel of natural gas. A 2009 study by the Argonne National Laboratory found that in the year 2007, at least 56 million barrels a day of contaminated wastewater were produced onshore in the U.S. as a byproduct of drilling for oil and gas. That calculates to more than 800 billion gallons per year. As the number of gas wells drilled since 2007 has increased exponentially, the amount of contaminated flow-back water has also increased.
Standard disposal options for flow-back water include discharge into surface water or underground injection. Both options pose a threat to the environment due to possible contamination of aquifers (Geochemical evidence for possible natural migration of Marcellus Formation brine to shallow aquifers in Pennsylvania, N. R. Warner et al., Proceedings of the National Academy of Sciences 109:11961-6, July 2012) and the risk of seismic activity (Two-year survey comparing earthquake activity and injection-well locations in Barnett Shale, Texas, C. Frohlich, Proceedings of the National Academy of Sciences 109:13931-8, August 2012).
Reusing untreated flow-back water for hydraulic-fracture is not viable due to the large potential these waters have in fouling or scaling underground geologic formations, as well as the pumping and drilling equipment, which then impedes the production of hydrocarbons. The flow-back water must therefore be filtered and treated before it can be reused in hydraulic fracture or reintroduced into the environment. For this reason, the flow-back water is typically trucked from the well site to a filtering location, and then trucked from the filtering location to its next destination. The process of using trucks to transport the water increases traffic volume on the roads and requires fossil fuels to power the trucks. Similarly, construction of piping systems to pump the water to and from a filtering location adds considerable expense. Once filtered and treated, a large amount of solid waste byproduct is generated which must then be trucked to landfill sites. It has been estimated that for every 100,000 gallons of flow-back water filtered and treated, 45,700 lbs. of potentially hazardous sludge is produced.
Fracturing fluids are in close contact with the geological formation during hydraulic fracture and thus pick up numerous materials from the rock. Such materials may include at least ions, metals and trace elements such as calcium, iron, magnesium, sodium, chloride, barium, mercury, arsenic, and lead. As such, typical flow-back water may contain contamination levels of salts, metals and dissolved solids (total organic and inorganic dissolved solids) that are as much as several thousand fold over the EPA's maximum allowable level.
Table 1 shows analytical data on hydraulic fracture flow-back water sampled from the Marcellus Shale geological area. Any process or system for the treatment and reuse of such flow-back water must remove the large quantity of salts and metals that act to foul the hydraulic fracture process and which are toxic to the environment. One of the most difficult (and toxic) contaminants to remediate in flow-back water from Marcellus Shale wells is barium. Water-soluble barium compounds are poisonous. At the high concentrations found in flow-back water, barium ions affect the nervous system, causing tremors, weakness, anxiety, shortness of breath and paralysis through the ion's ability to block potassium ion channels. Barium ions also affect the immune system, respiratory system, skin, and eyes, causing, for example, blindness.
TABLE 1Flowback WaterEPA's MaximumMultiples ofChemistry (mg/L)Contaminant LevelContaminationBarium18,21929,109×  Calcium15,5801.311,984×  Iron57.50.3191×Magnesium1,1391.3876×Strontium4,75041,187×  Hardness43,549—Extremely hard
While not as toxic as barium, strontium does have deleterious effects on the environment at the high concentrations found in flow-back water. For example, at high uptake levels, strontium has been found to cause disruption of normal bone development, anemia and oxygen shortages, and at very high concentrations to cause cancers and damage to the genetic materials in cells.
The extremely high hardness levels of the flow-back water are indicative of high concentrations of total dissolved salts. While low levels of certain salts such as calcium and magnesium in aquifers are crucial for the growth and development of wildlife and fish in the Marcellus geologic area, high concentrations can be extremely deleterious. The effect of high-salt loads on watersheds has been extensively documented through the study of road salt effects, and aquatic ecosystem impacts can be significant and far-reaching. Beyond direct toxicity to aquatic life, salinity affects the structure and function of aquatic ecosystems. High chloride levels, for example, have been known to be associated with the invasive golden algae (Prymnesium parvum). A Prymnesium bloom was responsible for the loss of all gill-breathing organisms in 26 miles of Dunkard Creek in southwestern Pennsylvania in the fall of 2009. High bromide levels, while non-toxic salt compounds, have been shown to react with disinfectants used at municipal water treatment plants to form brominated trihalomethanes (THM) which are volatile organic liquid compounds that become part of the drinking water. Studies have demonstrated a link between ingestion of THMs and several types of cancer and birth defects.
The present invention overcomes many of the shortcomings of the prior art treatment of flow-back water by providing a cost effective process and system to recycle all of the products in flow-back water.